1. Field of the Invention
The present invention relates to an electronic oscillator circuit, and to a method for operating such a circuit. More in particular the invention relates to an electronic oscillator circuit with a reduced temperature sensitivity.
2. Description of Related Art
Electronic oscillator circuits generally comprise an element that causes an oscillation, or that can be brought in resonance, and then causes an oscillation. These oscillations can be used as a frequency or time reference in electronic circuits. The higher exactness that is required, the higher accuracy requirements are set to the oscillation circuit.
Since the accuracy of the oscillator circuit is highly dependent on the accuracy of the oscillator or resonator element, frequency and time references according to the art are built on material properties that are very well implicitly defined, such as for Caesium and Rubidium oscillators, which get their frequency accuracy from atomic energy levels. However, these elements are relatively expensive.
On a lower accuracy level quartz crystals are available as cheaper references. Quartz crystals are easy to use in an electronic circuit, and their purity is quite high when the quartz crystals are found in nature, resulting in good resonance characteristics. In production in a melt the purity can get close to perfection, making a number of undesirable effects (noise and aging related) close to non-existent. Driving a quartz crystal is simple as quartz shows piezoelectric behaviour, meaning that the accurate mechanical vibration properties of a crystal are easily driven and read by electronics.
An alternative for such lower accuracies is also available, stemming from the production of integrated circuits: Micro Electrical Mechanical Systems (MEMS) allow for components that can vibrate mechanically while being integrated with electronics.
Both quartz crystal and MEMS resonators show limitations in their stability, especially in their temperature behaviour, which has accuracies in the order of tens of parts per million (ppm) for a relevant temperature range from −40 to +125 degrees Celsius, which does not compare very favourably to the accuracies of Caesium and Rubidium oscillators (order a few parts per trillion, ppt) or so called particle fountains (order 0.001 ppt).
According to the art it is known to compensate the temperature behaviour of mainly crystal resonators. At manufacturing time the temperature deviation of resonators and oscillator circuits is established and stored as a compensation table, which is used to correct the deviation. A compensation method is known which uses a separate temperature measurement and tries to compensate the related crystal frequency deviation with extra capacitance. However, if the measurement device is glued to the crystal it introduces new temperature effects in the crystal, and if not there will be a temperature difference between the two elements which will make the dynamic response limited. In either case the accuracy suffers.
Aging is a complicating factor here, but it can remain limited with pre-aging, and working with pure materials in general. When using MEMS, purity is implicit in manufacturing and hermetic sealing can be achieved at limited cost.
By compensation of that temperature behaviour the crystal and MEMS resonators are moving towards the range of a few parts per billion (ppb).
It is a goal of the present invention to provide a electronic oscillator circuit and a method for operating such a circuit that lack the above disadvantages.